This invention relates to a process for producing aqueous, high solids emulsions of epoxy resins, which are not solvent based, and which are suitable as impregnating resins for use in the preparation of laminates, for example, printed circuit boards.
State and Federal air pollution control regulations have become increasingly stringent, and could significantly affect the use of solvent-based epoxy compositions for industrial use. Other than regulations, the disadvantages of solvent-based epoxy compositions include solvent evaporation costs, the possible toxicity of some of the residual solvent retained in the cured material, and the cost and availability of solvents useful for epoxy dilution. To overcome the problems associated with solvent-based epoxy compositions, many systems have been converted to completely aqueous systems. These epoxy formulations contain a multitude of components which are necessary to obtain the desired properties.
Hosoda, in U.S. Pat. No. 3,983,056, recognizes air pollution problems caused by solvents, and discloses a room temperature curable aqueous epoxy resin paint, obtained by combining a mixture of bisphenol A epoxy resin with a novolac, dimer or trimer acid epoxy type, or a methyl substituted bisphenol epoxy, 0.1% to 20% by weight, based on epoxy, of a nonionic surface active agent, preferably a polyoxyethylene benzylated phenyl ether, preferably a polyamide curing agent, and a molybdic acid salt. Hosoda, in U.S. Pat. No. 4,073,762, teaches the use of a polyamide curing agent derived from a dimer acid and a polyether diamine to be used with the paint composition disclosed above. The combination of these compounds is essential to obtain this emulsified paint. The paint is used, for example, as a rust preventive primer.
Zentner, in U.S. Pat. No. 4,222,918, teaches the use of dicyandiamide dissolved in water as a curing agent for an emulsion of epoxy, anionic polycarboxylic emulsifying agent, and thermoplastic resin, such as polyvinyl formal or polysulfone. However, dicyandiamide has limited solubility in aqueous systems and is not very effective in the preparation of higher weight solids water emulsions. The use of a dicyandiamide derivative, such as monomethylol dicyandiamide, helps eliminate the solubility problem and enables one to prepare a wider range of emulsions. The preparation of these compounds, in organic solvents, is disclosed by Alvino et al., in U.S. Pat. No. 4,327,143. This solvent based system is not able, however, to utilize energy efficient infrared "B" staging operations without potential flammability problems. Substitution of water for solvent in the Alvino et al. compositions, to decrease potential flammability problems, may result in unstable mixtures with phase separation of water.
The need for energy efficient and non-polluting epoxy based high solids resin systems, that are also non-flammable, stable in and compatible with water, and possess suitable electrical and mechanical properties required by certain laminates, such as printed circuit boards, has not been met by these prior art compositions.